1. Field of the Invention
The present invention relates in general to a radio-frequency communication device operable to transmit and receive information to and from a desired communication object, and more particularly to techniques for eliminating an influence of a transmitter side of the device on a receiver side of the device.
2. Description of the Related Art
In various fields of data communication, there are used radio-frequency communication devices arranged to transmit transmitted signals toward desired communication objects, and to receive received signals transmitted from the communication objects in response to the transmitted signals, for thereby effecting communication with the communication objects. As one form of the radio-frequency communication devices, there is known a radio-frequency tag communication device (interrogator) operable for radio communication with small-sized radio-frequency tags (transponders) which store desired information. These radio-frequency tags and radio-frequency tag communication device constitute a so-called RFID (radio-frequency identification) system wherein articles to which the radio-frequency tags are affixed are identified by non-contact information reading and writing through electric waves. The information stored in the communication objects in the form of the radio-frequency tags can be read out by radio communication of the radio-frequency tag communication device with the radio-frequency tags, even where the radio-frequency tags are soiled or located at invisible places. The RFID system is expected to be used in various fields such as management and inspection of commodities.
The radio-frequency tags are labels each having a radio-frequency circuit element bonded thereto. The radio-frequency circuit element includes an IC circuit portion storing desired tag information, and an antenna which is connected to the IC circuit portion and arranged to transmit and receiver information. When a transmitted wave is transmitted from a transmitter antenna of a reader/writer functioning as the interrogator or radio-frequency tag communication device, toward the radio-frequency tag functioning as the transponder, the radio-frequency circuit element of the radio-frequency tag which has received the transmitted wave is activated with an energy of the received transmitted wave to transmit a reply wave. Namely, the reader/writer receives through its receiver antenna the reply wave from the radio-frequency tag as soon as the reader/writer has transmitted the transmitted wave. In this respect, it is noted that the amounts of attenuation of electric waves transmitted and received through the transmitter and receiver antennas (degree of separation of the transmitted and received waves or signals) are limited, so that a part of the transmitted wave is inevitably received by the receiver antenna, as an interfering leakage signal mixed in the reply wave transmitted from the radio-frequency tag. Thus, the transmitted wave or signal disturbs the reception of the reply wave or signal.
To solve the problem of the interfering leakage signal described above, it has been proposed to generate a cancel signal (compensating signal) for offsetting or canceling the unnecessary interfering leakage signal to be mixed in the reply wave, upon transmission of the transmitted wave, and to combine together the generated cancel signal and the unnecessary interfering leakage signal. JP-8-122429 A discloses an example of this conventional technique, wherein a device to identify mobile objects is provided with an interference compensating device. This interference compensating device includes a variable phase shifter and a variable attenuator for adjusting the phase and amplitude of the leakage signal which is a part of the transmitted wave, and a multiplexer for combining together a cancel signal generated by the phase and amplitude adjustment of the leakage signal, and the input to the receiver antenna, for thereby offsetting or canceling the unnecessary leakage signal. To generate the cancel signal, the variable phase shifter and the variable attenuator are manually adjusted so as to minimize the level of a composite signal obtained by combining together the cancel signal and the leakage signal which are generated while no reply wave is received from the transponders in response to the transmitted wave. To deal with a change of the unnecessary leakage signal changes due to aging deterioration of the variable phase shifter and attenuator, these variable phase shifter and attenuator are manually re-adjusted at a regular interval, for example, once per year.
There have been proposed techniques for enlarging the area of communication of the radio-frequency communication device. JP-5-128289 A discloses an example of such techniques, namely, a millimeter wave information reading system. This system uses an array antenna consisting of a plurality of antenna elements, and is arranged to control the phase of the transmitted signal transmitted from each of the antenna elements, and the phase of the received signal received by each antenna element, that is, to effect phased-array processing for transmission of the transmitted signal and reception of the received signal.
However, the conventional techniques described above have the following problems, in view of increasing introduction of the RFID system into various fields of industry. Namely, in the field of searching for desired articles of commodity within a warehouse, for example, radio communication between the interrogator and the transponders is adversely influenced by moving persons or metallic bodies, or existence of metallic bodies. Described in detail, the increasing introduction of the RFID system into the various fields of industry causes an increase of an influence of a change of the operating environment of the RFID system on the communication between the interrogator and the transponders, resulting in a considerable change of the condition in which the unnecessary leakage signal is generated from the transmitted signal or wave. According to the conventional technique wherein the cancel signal is optimized by manual adjustment of the variable shifter and attenuator, only a regular re-adjustment of the variable phase shifter and attenuator can be relied upon to deal with a change of the unnecessary leakage signal after the optimization of the cancel signal. Thus, it has been difficult to deal with a change of the operating environment of the RFID system in a real-time fashion, for sufficiently canceling the interfering leakage signal with the suitably adjusted cancel signal, to maintain a high degree of reception sensitivity of the interrogator or radio-frequency communication device.
The conventional techniques have another problem that the influence of the transmitted signal on the reception of the received signal changes each time the direction of transmission of the transmitted signal is changed. That is, a part of the transmitted signal transmitted from the radio-frequency communication device is mixed as a leakage signal into the received signal received by the radio-frequency communication device, so that the communication between the radio-frequency communication device and the communication object is adversely influenced by the leakage signal. Therefore, the leakage signal must be eliminated. However, the change of the direction of transmission of the transmitted signal causes a change of the influence of the leakage signal on the received signal, so that it is difficult to sufficiently eliminate the influence of the leakage signal. Namely, there has been a need of developing a radio-frequency communication device which can eliminate an influence of the transmitted signal on the reception of the received signal, according to a change of the direction of transmission of the transmitted signal.